The invention relates to a sealing device for a rotary feedthrough as is used, for example, in vacuum-processing facilities for coating substrates.
Rotary feedthroughs are needed in order to feed rotating parts (such as, for example, shafts) through housing walls and the like when the driving machine element, e. g. a drive device, is disposed on one side of the housing wall and the machine element to be driven, e. g. a rotating target, is disposed on the other side of the housing wall.
If a pressure differential must be maintained between the two sides of the housing wall (for example, atmospheric pressure on one side, high vacuum on the other side) and/or the atmospheres on the two sides of the housing walls have different compositions (for example, air on one side, inert gas on the other side), then it is necessary to configure the rotary feedthrough so that one prevents an undesired equalization of pressure or an exchange of gas between the two sides of the housing wall due to leakage in the rotary feedthrough.
Rotary feedthroughs for vacuum facilities can, for example, comprise two seals acting in tandem, where one seal is disposed so that it seals towards the atmosphere and the other seal is disposed so that it seals towards the vacuum or the process atmosphere. Between these two seals a complete separation of the media can be achieved, for example, with a sealing medium, that is, a sealing gas or a sealing liquid. Alternatively, separation of an atmosphere and a processing space can be achieved with an intermediate vacuum generated between both seals.
It is known that at higher rate of flow of alternating current through the shaft all the electrically conductive components which encircle the shaft in the manner of a ring can heat up to the point of destruction due to self-induction. This has as a consequence the fact that, in the selection of seals, springs, or supporting rings which encircle the shaft, attention must be paid to the fact that they have to be of non-conductive material or interrupted in their circumference.
It is also known that the sealing materials to be used must have good sliding properties in dry operation. This requirement can, for example, be met with the material PTFE with portions of graphite, molybdenum disulphide, or bronzes. The shaft surface can, for example, be coated with chromium oxide, which, in given cases, can be sealed with phenolic resin, thus attaining a very high strength of sealing.
However, several vacuum processes using large amounts of oxygen require a seal free of sealing media and cannot consist of oxidizing materials. For these instances of use, the sealing material PTFE with portions of polyoxybenzoyl ester combined with a countersurface of chromium oxide sealed with phenolic resin has proven itself effective. In that case the seal runs on the chromium oxide layer without lubrication.
In order to achieve a sufficient sealing effect, sealing elements of the sealing device can be provided, for example, with a sealing lip. Such sealing lips can, for example, be generated by using as a sealing element a circular ring disc whose inner diameter is less than the outer diameter of the rotating machine element which projects through the rotary feedthrough. Along with this there is a sealing gap between the sealing lip and the rotating machine element, said sealing gap being under intrinsic tension.
The atmospheric pressure which is present and the intermediate vacuum which is present generate in addition a pressure load on the sealing lip. Under the different pressure loads of the two sealing lips, different wear of the same arises towards the sliding surface. However, wear is of decisive importance for the service lifetime of the sealing device.
In the selection of the sealing materials of the sealing lips one must pay attention to the properties of good restoration properties, low wear, and good sliding characteristics. Problematic in the selection of the sealing materials is the fact that all the properties have mutual interactions.
The envisioned sealing combinations have the disadvantage that the sealing lip disposed nearer to the vacuum, due to the lower forces of compression, produces a lower sealing action and the sealing lip disposed nearer to the atmosphere experiences higher wear.